Membrane proteins represent among the most challenging protein targets for structure determination and yet also the most important for human health. Major emerging advances in X-ray sources, including the use of X-ray free electron laser (XFEL) diffraction, tightly focused synchrotron XRD, and improvements in electron diffraction (ED) instrumentation have inexorably reduced the crystal sizes required for structure determination into the protein nanocrystal regime. These trends are increasingly leading to a major bottleneck in protein nanocrystal screening, the importance of which will only increase as these new diffraction capabilities continue to advance. The primary objective of the proposed effort will be to develop measurement tools based on nonlinear optical interactions from ultrafast laser sources for selectively screening protein nanocrystal formation. Three key novel approaches will be developed. In the first, polarization-dependent TPE-UVF will be developed and validated as a complement to conventional SONICC for the detection of immobile protein crystals, such as those encountered in lipidic mesophases and under cryogenic conditions. Ordered crystalline domains produce polarization-dependent TPE-UVF patterns distinct from aggregates and solvated proteins. In the second, SHG and TPE-UVF autocorrelation methods will be developed for improving the detection limits of diffusing nanocrystals, targeting 2D and 3D nanocrystals generated from aqueous solutions. Autocorrelation relies on analysis of the combined weak responses from many nanocrystals for signal to noise enhancement, while simultaneously providing information on translational diffusion. Finally, nonlinear optical cross-correlation spectroscopy methods will be advanced using polarization-dependent SHG and TPE-UVF for the selective detection of protein nanocrystal rotational dynamics. The correlations within the polarization-dependence of TPE-UVF will preserve crystal-specific detection by both TPE-UVF and SONICC for freely diffusing nanocrystals. Validation of the proposed approaches will be performed using a combination of focused XRD, XFEL diffraction, and ED, with diverse and representative membrane protein nanocrystal samples provided through collaboration with Vadim Cherezov (Scripps) for lipidic mesophase crystallizations, Petra Fromme (U. Arizona) for crystals amenable to XFEL analysis, and David Stokes (NYU) for 2D nanocrystals for structure determination by ED. Combined SONICC and synchrotron XRD measurements will be performed at the APS using a prototype capable of combined SHG imaging and focused minibeam XRD. Although the primary purposes of the present studies are to fundamentally advance membrane protein nanocrystal detection, close collaboration with Formulatrix will help lower the barriers to rapidly transition he findings of these inquiries to high-throughput commercial platforms for routine protein nanocrystal detection should the proposed efforts prove successful.